Whole-cell sensing systems employ genetically engineered living cells that contain biospecific recognition elements for the detection of analytes of interest. In bacterial operon-based whole-cell sensing systems, the sensing element is comprised of a regulatory gene encoding a regulatory protein and a specific operator/promoter (O/P) sequence of DNA. The regulatory protein is capable of recognizing the analyte and controlling expression of a reporter gene that is placed under transcriptional control of the O/P. Upon binding the target analyte, the regulatory protein activates gene transcription, with subsequent expression of the reporter protein leading to the generation of a detectable signal. The reporter gene is expressed in a concentration dependent manner and calibration plots can be constructed by relating the signal generated with the concentration of analyte.
Whole-cell sensing systems have been employed in a variety of environmental bioassays, as well as in biotechnology, pharmacology, and clinical chemistry applications. Numerous whole-cell sensing systems have been developed for environmental monitoring purposes. These include biosensors for the detection of toxic compounds such as mercury, arsenic, cadmium, lead, zinc, and several organic pollutants present in different types of environmental samples.1-3 Recently, they have also been used for monitoring the bioavailability of chemicals such as nitrogen, phosphorus, and carbon in soil.4 Additionally, whole-cell sensing systems have been developed for the detection of biomolecules, including sugars, drugs, and quorum sensing signal molecules.5-7 Various aspects of the construction of genetically engineered microorganisms and their application as biosensors in various fields have been discussed in recent reviews, which are incorporated herein by reference in their entireties.1,3,8-10 These bacterial sensing systems can provide an inexpensive and simple way to selectively, sensitively and rapidly detect very low levels of analytes. Additionally, they can supply important information about the bioavailability and activity of the analyzed compounds.
Although these systems and methods are promising, improvements are needed, especially for long-term storage in unfavorable conditions. Accordingly, there remains a need in the art for development of whole-cell biosensors that function satisfactorily after long-term storage, even under harsh environmental conditions.